Neutron detectors play an important role in downhole tools. At present, two kinds of neutron detectors are used in downhole tools. One type of detector serves to detect fast neutrons and may employ a plastic scintillation detector. A second, more common type is a detector of thermal or epithermal neutrons such as a 3He detector or, less frequently, a scintillation detector using 6Li-glass. 3He detectors are excellent detectors of thermal or epithermal neutrons and they are virtually insensitive to gamma-rays. 6Li-based detectors on the other hand have significant gamma-ray sensitivity and suppression or subtraction of gamma-ray induced background in the presence of gamma-rays from inelastic neutron interactions or neutron capture is difficult and inaccurate.
Neutron detection is used in a multitude of downhole tools. The basic application is in the measurement of neutron porosity through the detection of thermal or epithermal neutrons. Other applications may include the determination of neutron-gamma-density (see U.S. Pat. Nos. 5,608,215 and 5,804,820, assigned to the assignee of the present disclosure). In addition, the present scarceness of 3He, a gas which is widely used in thermal and epithermal neutron detectors, has made alternatives for neutron detection to be of great interest.